The guided wave and the TDR (time domain reflectometry) measurement method is one of a variety of measurement methods to determine the level of a medium in a process container, such as a tank in an industrial process. Generally, in the TDR measuring method a signal is emitted along a surface waveguide or coaxial waveguide that extends into the process medium. A change of dielectric constant at the surface of the medium surrounding the waveguide partially reflects the signal back along waveguide. From the time difference between the emission of the emitted signal and the reception of the reflected echo signal, the level of the medium in the container can be determined.
For process level measurement, generally, a rod or cable-like waveguide protrudes into the container or vessel, usually through a nozzle, often referred to as a stand-off, into the medium. In some cases the wave guide is installed into a still well, which is a pipe installed inside the vessel that extends to or near the bottom. The waveguide may be quite long in applications where the process container is rather tall or the stand-off of the container is particularly tall. Important for the reproducibility and reliability of the measurement, the rod or cable-like waveguide must not touch the vessel wall, stand off or the stilling well. A stilling well can prevent the waveguide being pushed out of its inherently desirable, preferably vertical position in containers due to turbulence and currents in the process medium. In such process conditions, the position of the waveguide relative to the inner wall of the container may be changed. Moreover, a change in position of the waveguide caused by installations that are not vertical or highly turbulent process conditions can result in the probe touching the side of the stilling well thus shorting out the signal.
Similar problems have been encountered with long probes used in capaciatancemeasurement systems. The probe of a capacitance transmitter also must not touch the wall of the vessel, stilling well or stand-off to ensure proper reliably of the system.
An uncontrolled change in position of the waveguide can cause variation in the measurement results such that the results are not comparable with the actual level. If the waveguide contacts an inner wall of the container, the stand-off or the stilling well, an accurate determination of the medium level is no longer possible because the emitted signal may be short circuited or the reflected echo signal may no longer be from the true surface of the medium. In highly agitated media due to, for example, filling processes, emptying processes, mixers or agitators operating in the container, the medium can move waveguide from its mostly vertical position, even within a stilling well, which can foil reproducible measurements. Another problem associated with motion in the medium acting on the waveguide is mechanical stress on the waveguide, which can cause damage over time.
For these reasons, spacers may be attached to the waveguide to hold the waveguide in its desired vertical position with respect to the inner wall of the container, the stand-off and/or the stilling well. However, conventional spacers are generally expensive because they are commonly made of a material that requires machining and because they often require multi-piece assemblies to attach securely to the waveguide. Moreover, conventional spacers can interfere with the level measurement due to pooling of the medium on top-facing surfaces of the spacer. This pooling of the process media can result in the radar signal being reflected from the spacer, resulting in an incorrect reading. Accordingly, there remains a need for further contributions in this area of technology.